Mark duplicate files even when scan result found no new files.

ui/file_manager/file_manager/foreground/js/import_controller.js

Index: ui/file_manager/file_manager/foreground/js/import_controller.js
diff --git a/ui/file_manager/file_manager/foreground/js/import_controller.js b/ui/file_manager/file_manager/foreground/js/import_controller.js
index a217242890392d25874cde149bac5da6085383de..e25e6b0537bc9969e1c60407d610b0dbb3b7cdc3 100644
--- a/ui/file_manager/file_manager/foreground/js/import_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/import_controller.js
@@ -282,13 +282,12 @@ importer.ImportController.prototype.onClick_ =
 };
 
 /**
- * Executes import against the current directory. Should only
- * be called when the current directory has been validated
- * by calling "update" on this class.
+ * Executes import against the current context. Should only be called
+ * after the current context has been validated by a scan.
  *
  * @private
  */
-importer.ImportController.prototype.execute_ = function() {
+importer.ImportController.prototype.startImportTask_ = function() {
   console.assert(!this.activeImport_,
       'Cannot execute while an import task is already active.');
 
@@ -308,6 +307,17 @@ importer.ImportController.prototype.execute_ = function() {
   };
   var taskFinished = this.onImportFinished_.bind(this, importTask);
   importTask.whenFinished.then(taskFinished).catch(taskFinished);
+}
+
+/**
+ * Executes import against the current context. Should only be called
+ * when user clicked an import button after the current context has
+ * been validated by a scan.
+ *
+ * @private
+ */
+importer.ImportController.prototype.execute_ = function() {
+  this.startImportTask_();
   this.checkState_();
 };
 
@@ -375,7 +385,15 @@ importer.ImportController.prototype.checkState_ = function(opt_scan) {
   }
 
   if (opt_scan.getFileEntries().length === 0) {
-    this.updateUi_(importer.ActivityState.NO_MEDIA);
+    if (opt_scan.getDuplicateFileEntries().length === 0) {
+      this.updateUi_(importer.ActivityState.NO_MEDIA);
+      return;
+    }
+    // Some scanned files found already exist in Drive.
+    // It means those files weren't marked as already backed up but need to be.
+    // Trigger sync for that purpose, but no files will actually be copied.
+    this.startImportTask_();

[Steve McKay, 2016/12/27 17:10:58]

Seems like this might result in some false reports of imports. Can we break up
startImportTask_ to avoid this?

Also, it looks like startImportTask_ will ultimately call onImportFinished_
which itself calls checkState_ again. So there appears to be risk of an infinite
loop.

[yamaguchi, 2017/01/05 11:50:04]

I agree. I found it's importer.MediaImportHandler.ImportTask that sends reports
to the tracker.
Therefore I think a possible fix would be to stop reusing ImportTask but
establish a different type of Task (e.g.
importer.MediaImportHandler.MarkDuplicatesTask) sharing part of the code.

https://cs.chromium.org/chromium/src/ui/file_manager/file_manager/background/...

Having said that, this code path will no longer be used after the change to skip
content-duplicate check at the scan phase. It's because history-duplicate is not
included in the getDuplicateFileEntrie().
https://cs.chromium.org/chromium/src/ui/file_manager/file_manager/background/...

I think we need to cleanup code around this.
Thanks for finding it out.
I agree that we should have a separate code path to avoid such risk.

+    this.updateUi_(importer.ActivityState.IMPORTING, this.activeImport_.scan);
     return;
   }